Spectroscopy is a general term for the process of measuring energy or intensity as a function of wavelength in a beam of light or radiation. Many conventional spectroscopes, and components comprising a spectroscope system, also referred to as an instrument, may include basic features and components such as a slit and a collimator for producing a parallel beam of radiation, one or more prisms or gratings for dispersing radiation through differing angles of deviation based on wavelength, and apparatus for viewing dispersed radiation. Spectroscopy uses absorption, emission, or scattering of electromagnetic radiation by atoms, molecules or ions to qualitatively and quantitatively study physical properties and processes of matter.
Light or radiation directed at a target, or sample, during operation of a spectroscope system may be referred to as incident radiation. Redirection of incident radiation following contact with a sample of physical matter ("sample") commonly is referred to as scattering of radiation. To the extent that atoms or molecules in a sample absorb all or a portion of incident radiation, rather than reflect incident radiation, a sample may become excited, and the energy level of the sample may be increased to a higher energy level. Electromagnetic radiation, including incident radiation, that passes through a sample may produce a small portion of light that is scattered in a variety of directions. Light that is scattered but continues to have the same wavelength as the incident radiation may also have the same energy, a condition often referred to as Rayleigh or elastically scattered light. Incident radiation that is scattered during a change of vibrational state in molecules may be scattered with a different energy, and such scattered light may be called Raman scattered light. Such phenomena have been used in conjunction with spectroscopy to qualitatively and quantitatively study physical properties and processes, including identification of chemical properties, compositions, and structures of a sample.
A wave associated with electromagnetic radiation may be described by wavelength, the physical length of one complete oscillation, and by frequency of the wave, the number of oscillations per second that pass a point. If incident radiation is directed at a sample, the wavelength of the incident radiation ("incident wavelength") may remain substantially unchanged in scattered radiation. Alternatively, if incident radiation is directed at a sample, the wavelength in the scattered radiation may acquire one or more different wavelengths than the incident wavelength. The energy differential between the incident radiation and the scattered radiation may be referred to as a Raman shift. Spectroscopic measurement of Raman scattered light seeks to measure the resulting wavelength of such scattered light.
Raman scattered light may occur at wavelengths shifted from the incident light by quanta of molecular vibrations. The phenomenon of Raman scattered light, therefore, is useful in spectroscopy applications for studying qualities and quantities of physical properties and processes, including identification of chemical properties, compositions, and structure in a sample. Currently, Raman shift spectroscopic analytical techniques are used for qualitative and quantitative studies of samples. If incident radiation is used to scatter light from a sample, and scattered radiation data is measured, the scattered radiation may provide one or more frequencies associated with the sample, as well as the intensities of those shifted frequencies. The frequencies may be used to identify the chemical composition of a sample. If, for example, intensities are plotted on a Y-axis, and frequency or frequencies are plotted on an X-axis, the frequency or frequencies may be expressed as a wave number, the reciprocal of the wavelength expressed in centimeters. The X-axis, showing frequency or frequencies, may be converted to a Raman shift in wave numbers, the measure of the difference between the observed wave number position of spectral bands, and the wave number of radiation appearing in the incident radiation.
While these principles and phenomena are known, efforts to apply the principles and phenomena to qualitative and quantitative analyses of samples has not resulted in uniform, predictable results, or in acceptable levels of precision and accuracy of Raman spectra. Because of instrumentation variabilities, inherent weakness of a Raman scattered signal, fluorescence, and other limitations associated with spectroscopy instruments, the goal of producing a standard Raman spectrum for use in sample analyses has proven to be a challenge not achieved by apparatus and methods known in the art.
For example, spectroscopic measurements of Raman scattered light seeking to measure wavelength or intensities, or both, of scattered light, may be affected by the instrument, or spectroscopic system, itself. A number of components of an instrument may contribute individually and collectively to undesirable instrumentation variabilities that affect spectral data measured by the instrument. Thus, Raman scattered radiation from a sample may be observed, measured, and directed through an instrument by optics of a spectrometer, may be coded by a device such as an interferometer, and may be directed to one or more detectors to record Raman spectra. Any one, or all, of such components of a spectrometer system may induce or contribute to instrumentation variabilities that may reduce or adversely affect the precision and accuracy of measurements of Raman scattered light.
Further, Raman scattering is a comparatively weak effect when compared with Rayleigh or elastic scattering. Nevertheless, Raman scattering offers a significant opportunity for qualitative and quantitative studies of physical properties and processes, including identification of chemical compositions and structure in samples of physical matter. To appreciate these phenomena, as well as understand the problems solved by the present invention, it should be noted that depending on the compound comprising a sample, only about one scattered photon in 10.sup.6-8 tends to be Raman shifted. Because Raman scattering, therefore, is such a comparatively weak phenomenon, a spectrometer used to disperse radiation for measurement purposes should have minimal stray light and be able to substantially reject Rayleigh scattering. Otherwise, a Raman shift may not be measurable. In addition, because multiple lines in the frequency of the source of incident radiation may cause shifted, multiple sets of spectra from a sample, conventional Raman experimentation discloses that a source or sources of incident radiation that causes excitation in a sample used in connection with a spectrograph should be substantially monochromatic, preferably providing a single frequency or wavelength. Recognition that the source of incident radiation requires a substantially monochromatic frequency has led to use of a variety of laser light sources as a source of incident radiation because of the substantially monochromatic frequency and high intensity of a laser. Gas lasers such as helium--neon, helium--cadmium, argon-ion, krypton-ion, as well as solid state lasers including Nd-YAG, and diode lasers, solid state tunable lasers, liquid dye lasers, and other lasers, have been used in spectroscopy apparatus seeking to measure Raman spectral data, including wavelength.
Preferably, a source of incident radiation would provide a substantially monochromatic frequency and radiation closer to the blue portion of the visible light spectrum providing short wavelength excitation because the Raman effect is enhanced by use of short wavelength excitation, and because of the enhanced quantum efficiency ("QE") of charged coupled detectors ("CCD's") in use today. Indeed, an undesirable result of incident radiation on a sample occurs if a sample generates red shifted radiation as part of a radiation absorption process, a phenomenon commonly referred to as fluorescence. Fluorescence occurs when absorbed radiation is lowered in frequency by internal molecular processes and emitted as radiation that is closer to the red end of the visible light spectrum. Fluorescence sometimes may be strong enough in comparison with the Raman shift to swamp, or substantially eliminate, the weaker Raman signal. Current technology, however, particularly in connection with silicon detectors, substantially restricts use of instrument components that tend to provide radiation far into the infrared ("IR") region of the light spectrum, rather than blue.
Instrumentation variabilities, however, are not the only limiting factors affecting current Raman spectroscopy analyses. While a laser often is suitable as a source of incident radiation, the frequency or frequencies associated with incident radiation from a laser may be unstable. Unstable frequencies of incident radiation are yet other variations that may affect measurements of a Raman signal. While lasers may provide a substantially monochromatic source of incident radiation, variations in the center wavelength and frequency of lasers, even those that are apparently similar, also may occur. To the extent that wavelengths of a source of incident radiation vary, the resultant Raman spectra may vary. Current commercial applications of Raman principles and phenomena are hindered by requirements for substantially constant monitoring of excitation wavelengths from the source of incident radiation to avoid erroneous data. Proposals for overcoming this limitation have included, therefore, use of two or more sources of frequency stabilized incident radiation, one as the primary or actual source, a second as a reserve or backup source. Use of two or more sources of frequency stabilized incident radiation, however, adds significantly to the costs of such a system. Use of an actual source of frequency stabilized incident radiation, with a reserve source of frequency stabilized incident radiation as a backup, resolves neither possible failure of both sources of incident radiation, nor undesirable variations of wavelength in both excitation sources. Costs associated with providing two or more sources of frequency stabilized incident radiation in an apparatus that employs only one source of frequency stabilized incident radiation also is comparatively high.
In addition, current spectrometer systems seeking to use a Raman shift in connection with qualitative or quantitative analyses of a sample may have to overcome limited sensitivity of one or more instrument detectors currently available for use in spectrometer systems. For example, a detector consisting of a silicon charge coupled device has a limited range of sensitivity in detecting wavelength. A practical upper wavelength for silicon charge coupled devices currently available is about 1.15 microns. Current detector sensitivity to wavelength is not constant from upper to lower limits. Detectors that appear to be similar may exhibit different sensitivity responses. Further, charge coupled detectors may have multi-channel capabilities, allowing a detector to accept spectra substantially simultaneously, or substantially at one point in time ("real time"). Some detectors, however, are capable of collecting information only over time, rather than in real time. The need for a detector to accept spectra substantially simultaneously, or at one point in time, is due in part to system drift that may be associated with the spectroscopy system itself, as well as with time differences occurring during data gathering.
Thus, present spectrometer technology indicates problems and limitations caused by spectrometry hardware components. The spectroscopic instrument may exhibit differences among similar or related components, or instrumentation variabilities, that may induce or cause differences in measurements of spectra from the same sample. As described, differences in measurements of spectra from the same sample may be caused by instrumentation variabilities including optics, detectors, and other components of a spectroscopy system. Efforts have been made to control the effect of instrumentation variabilities on qualitative and quantitative analyses performed using the Raman shift, but none has proven consistently successful. Further, none of the efforts has resulted in a cost effective spectroscopy system for measuring a Raman shift that achieves superior precision and accuracy in connection with Raman spectra, as well as instrument independence that results in a fully automated system.
For example, at least one system proposes use of a sample and a secondary reference material from which a spectrograph may gather intensity and wavelength information of and from an excitation source. Introduction of one or more secondary reference materials adds considerable complexity to an application of the Raman shift principles. The spectrum of a reference material must be known, and must remain constant during use. A reference material, however, may not provide consistent measurements because of variations in the reference material. Requiring use of a secondary reference material also results in loss of resolution in the sample because, for example, any Raman spectra of a sample may be broadened by spectral properties of the reference material. Significant broadening of a Raman spectrum will yield an inaccurate qualitative or quantitative analysis of a sample. Also, because significant portions of signals associated with sources of incident radiation may be used to obtain a spectrum or spectra of a reference material, the relationship between the signal and the noise shown in graphical readings portraying the data is reduced.
There are, therefore, a number of problems to solve to achieve the goal of providing an apparatus for measuring and applying instrumentation correction to produce a standard Raman spectrum that permits a user to make spectroscopic measurements using principles associated with Raman scattered light. The problems to be solved include providing an apparatus for measuring and applying instrumentation correction to produce a standard Raman spectrum. Another problem includes providing an apparatus that produces and achieves superior precision and accuracy in connection with measurements of Raman spectra, as well as instrument independence, and achieving such results by providing a fully automated system. The term "fully automated system" means a system that includes the capability of not requiring an operator of the system to be either skilled in the art or have special skills, yet is capable of maintaining the quality of the data over a time period unmonitored or unattended by an operator of the system. In addition, what should be achieved is high resolution Raman spectra using an apparatus and method of operation of the apparatus that is easy to use, predictably accurate, easy to practice, and relatively cost effective.
What is needed, therefore, is an apparatus for measuring and applying instrumentation correction to produce a standard Raman spectrum ("Raman apparatus"). The Raman apparatus should be capable of providing and achieving superior precision and accuracy in connection with Raman spectra. Also needed is a Raman apparatus that is instrument independent, while eliminating the need for ideal excitation frequency stability. Such a Raman apparatus also should be capable of producing a high resolution Raman spectrum unaffected by Rayleigh or other undesirable radiation scattering. Wavelengths and intensities should be determinable substantially simultaneously, in real time. The Raman apparatus should be able to provide from the source of incident radiation at least an incident beam and a monitor beam for purposes of analyzing incident radiation. The Raman apparatus also should provide high resolution of monitor beam measurements and data, using, as a nonexclusive example, one or more higher resolution dispersive optics such as one or more prisms or gratings. Also needed in the Raman apparatus is a detector or detectors capable of collecting spectral information as a whole, in real time, rather than over a time differential. Effects of instrumentation variabilities on the Raman spectra caused by characteristics of components of the Raman apparatus itself, should be compensated for. The Raman apparatus should provide a fully automated system, meaning that any variations in a source of incident radiation are determined and resolved in real time. The Raman apparatus also should not require use of a significant portion of the source of incident radiation in the process of determining characteristics of the source of incident radiation.
One of many advantages of the present invention, therefore, is a Raman apparatus for measuring and applying instrumentation correction to produce a standard Raman spectrum. An additional advantage of the present invention is a Raman apparatus that provides standard reference data from any spectrometer or similar instrument because instrumentation variabilities are rendered irrelevant. The present invention is capable also of achieving superior precision and accuracy in connection with Raman spectra, as well as instrument independence, while eliminating the need for ideal excitation frequency stability. The present invention, therefore, is capable of achieving superior precision and accuracy, while being independent of intensity fluctuations in connection with the source of incident radiation, and of frequency drifts in the source of incident radiation, as well as in the system itself. As indicated, the present invention also provides a system that is capable of being fully automated.
Another advantage of the present invention is the ability of the Raman apparatus to provide double dispersion of a monitor beam to acquire substantially simultaneously high resolution spectral data about a source of incident radiation and instrumentation variabilities. The present invention substantially simultaneously collects spectral data from a monitor beam and from a Raman beam, which originated from a sample and from a source of incident radiation, in terms of both spectral properties and intensity levels. It provides a detector having optimum detector sensitivity capable of accepting one or more dispersions, including double dispersion of a monitor beam, that enables the detector to collect spectral information as a whole, in real time, rather than over a time differential. The Raman apparatus also compensates for, or obviates, effects on Raman spectra caused by characteristics of components of the Raman apparatus itself. Use of one or more reference materials to obtain or generate data is not required. Another advantage of the present invention is its ability to enlarge the universe of both users and applications because of the several advantages of the present invention, including the fact that the apparatus and method of operation of the present invention respectively are easy to use and to practice, and cost effective for their intended purposes.
These advantages and other features of an apparatus for measuring and applying instrumentation correction to produce a standard Raman spectrum will become apparent to those skilled in the art when read in conjunction with the accompanying following description, drawing figures, and appended claims.